This invention relates to multi-turn coil structures, particularly miniature wire coil structures such as used in electronic circuits of security or "smart" cards, and to methods of winding same.
In a miniature multi-turn wire coil structure made by conventional methods, the coil is wound on a former outwardly from the center of the coil. The outer end of the coil can be extended in the general plane of the coil to form an incoming or outgoing lead. If the inner end of the coil, however, is to be extended to the outside to form an outgoing or incoming lead, it must occupy a plane adjacent the plane of the coil, so that the minimum thickness or width of such a coil is increased by the diameter of the wire.
When it is necessary to accommodate a relatively large number of turns in a limited outer diameter, the coil is wound in two adjacent planes with the winding crossing over between the planes in alternate turns. This results in the coil having a width or thickness of approximately twice the wire diameter. Moreover, the thickness is increased by an additional wire diameter since the inner end of the coil must be brought to the outside in a further adjacent plane.
In certain applications, thickness limitations are critical and it would be advantageous to provide a multi-turn coil including input and output leads which is of reduced thickness compared to known such coils. For example, in portable "smart" electronic cards, such as credit cards or cards used for accessing security systems and the like, there may be a need to accommodate miniature multi-turn wire coils within the thickness of a card layer, and reduction of coil thickness can contribute to an equivalent reduction of the overall card thickness.
Accordingly, the present invention seeks to provide a multi-turn coil structure with input and output leads which is of reduced thickness compared to known such structures.